1. Field of the Invention
The present invention relates to a defect detection method of a turbine generator end ring by ultrasonic testing.
2. Description of the Related Art
Ultrasonic testing of a turbine generator end ring is conducted to detect defects such as stress corrosion cracks arising on the surface of the end ring. The ultrasonic testing of the end ring is conducted in an assembled state without disassembling the end ring. The angle beam technique is generally applied to the ultrasonic testing of the end ring. In the ultrasonic testing in which the angle beam technique is applied, an angle beam probe is used to scan the surface of an end ring for defects. The portion of a defect is determined by detecting a defect echo, which is a reflected echo from the defect.
FIG. 11 is a schematic diagram showing a state of ultrasonic wave transmission/reception of an angle beam probe in ultrasonic testing by the angle beam technique. FIG. 12 is a waveform diagram showing a relationship between received input of the angle beam probe and a propagation time of an ultrasonic wave in the ultrasonic testing by the angle beam technique. In FIG. 12, the vertical axis represents the received input of the angle beam probe and the horizontal axis represents the propagation time (beam path length) of the ultrasonic wave. That is, FIG. 12 is displayed in an A-scan. FIG. 12 is shown in a direct current (DC) representation. The same reference numbers are attached to the same components as those in FIG. 11 or 12 and a detailed description thereof is omitted. Similarly, a detailed description is omitted below.
An angle beam probe 2 is installed on the surface of an outer circumferential surface of an end ring 1. In case a defect 4 is present on an inner circumferential surface of the end ring 1, when an ultrasonic wave beam 3 is incident from the angle beam probe 2, the ultrasonic wave beam 3 is reflected by the defect 4. A reflected wave reflected by the defect 4 is received by the angle beam probe 2. The defect 4 is, for example, a stress corrosion crack. At this point, as shown in FIG. 12, a transmission pulse 5 and a defect echo 6 are displayed in the A-scan screen.
In case the defect 4 is not present on the inner circumferential surface of the end ring 1, even though the ultrasonic wave beam 3 is incident from the angle beam probe 2, no reflected wave reflected on the inner circumferential surface is received by the angle beam probe 2. This is because there is no reflector on the inner circumferential surface of the end ring serving as a bottom. Thus, in case the defect 4 is not present, only the transmission pulse 5 is displayed in the A-scan screen and the defect echo 6 is not displayed.
By using the ultrasonic testing by the angle beam technique in this manner, the defect 4 can easily be detected. However, when inspecting defects of an end ring of a turbine generator, attention should be paid to detection of a false echo.
An example in which a false echo is detected in the ultrasonic testing by the angle beam technique will be described with reference to FIGS. 13 to 16. FIG. 13 is a schematic diagram illustrating detection of a false echo 8 by a shaft shrinkage fitting portion 7. FIG. 14 is a waveform diagram in the A-scan DC representation of detection of the false echo 8 by the shaft shrinkage fitting portion 7. FIG. 15 is a schematic diagram illustrating detection of the false echo 8 by a joint portion of a short-circuit ring 9 of the end ring 1. FIG. 16 is a waveform diagram in the A-scan DC presentation of detection of the false echo 8 by the joint portion of the short-circuit ring 9 of the end ring 1.
As shown in FIG. 13, the shaft shrinkage fitting portion 7 is provided on the inner circumferential surface of the end ring 1. The shaft shrinkage fitting portion 7 normally has a substantially rectangular section in contact with the end ring 1. The short-circuit ring 9 is arranged, as shown in FIG. 15, on the inner circumferential surface of the end ring 1 along a circumferential direction. The short-circuit ring 9 is normally arranged in the circumferential direction by being divided into a plurality of portions. Thus, an edge of one of the divided short-circuit ring 9 becomes a short-circuit ring joint portion with another of the divided short-circuit ring 9.
If the ultrasonic testing by the angle beam technique is conducted when an edge of the shaft shrinkage fitting portion 7 or a joint portion of the short-circuit ring 9 is present on the inner circumferential surface of the end ring 1, the following will occur.
The ultrasonic wave beam 3 incident from the angle beam probe 2 is reflected by the edge of the shaft shrinkage fitting portion 7 or the joint portion of the short-circuit ring 9 as a reflector. A reflected wave reflected by the reflector is received by the angle beam probe 2. As shown in FIGS. 14 and 16, the reflected wave becomes the false echo 8. When displayed in the A-scan, it is difficult to distinguish the false echo 8 from the defect echo 6 shown in FIG. 12.
A method described in Jpn. Pat. Appln. KOKAI Publication No. 11-287790 is known as a method of distinguishing defect echoes from false echoes. In the method according to this technology, the ultrasonic testing by the angle beam technique is first conducted by depth scanning. Next, split spectrum processing (SSP) of a testing signal obtained from the ultrasonic testing is performed to determine an indication length. Whether an indication is a defect echo or a false echo is interpreted based on the indication length.
However, the above interpretation method is applied to austenite stainless steel. Austenite stainless steel has larger crystal grains compared with ferrite material. Crystal grains of austenite stainless steel often become still larger at a boundary of welding or the like. This interpretation method is intended to interpret false echoes generated by an ultrasonic wave incident near a boundary of welding of such austenite stainless steel being reflected, refracted, or scattered. In contrast, false echoes generated when defects of a turbine generator end ring are detected do not result from the size of crystal grains of material. As shown in FIGS. 13 to 16, false echoes in a turbine generator end ring result from a structure such as the shaft shrinkage fitting portion 7 or a joint portion of the short-circuit ring 9 present on the inner circumferential surface of the end ring 1. Thus, it is very difficult to distinguish false echoes from defect echoes in a turbine generator end ring by using the interpretation method described in Jpn. Pat. Appln. KOKAI Publication No. 11-287790 or the like.
Thus, a defect of the turbine generator end ring 1 is interpreted in the following way. In case an indication echo is displayed in the ultrasonic testing by the angle beam technique, an internal structure diagram is further referenced. In case a structure such as the shaft shrinkage fitting portion 7 or a joint portion of the short-circuit ring 9 is present at a portion where the indication echo is displayed, the indication echo is presumed to be a false echo. In case such a structure is not present, the indication echo is presumed to be a defect echo. However, this interpretation method may not be able to sufficiently detect defects of a turbine generator end ring particularly in the vicinity of an internal structure.